1. Field of the Invention
The present invention relates to a wireless handset used in a communication system which switches a plurality of call channels for use, and more particularly to a wireless handset provided with a tunable antenna suitable for miniaturizing the wireless handset.
2. Description of the Related Art
There is a demand for more compact, thin wireless handsets from the viewpoint of improvement of portability. Although an antenna used in a wireless handset must have sensitivity throughout a frequency band of a system in which the handset is used, since self bandwidth decreases as the volume occupied by an antenna decreases, an attempt to miniaturize an antenna while maintaining bandwidth in an identical frequency band has been difficult.
Generally, the band of frequencies used for calls between a specific base station and terminal equipment is much smaller than an entire frequency band of a system. Accordingly, for each call, by adaptively changing a center frequency of impedance matching of an antenna to a frequency used for the call, a frequency band that the antenna should have can be decreased and the volume of the antenna can be reduced. As such an antenna, there is suggested by U.S. Pat. No. 6,034,644 a coaxial resonant slot antenna which supplies RF power to a strip conductor disposed within a conductive flat cubic with a slot provided on the top thereof and insulation from the conductive flat cubic, wherein the coaxial resonant slot antenna is a tunable slot antenna in which at least one island conductor is provided within the slot and center frequencies of impedance matching of the antenna can be changed in a wide range by changing capacitance values between the island conductor and the wall face of the conductive flat cubic.
If a center frequency of impedance matching of a tunable antenna such as said tunable slot antenna can be controlled so as to tune to a frequency used for a call, an antenna having a much smaller call band than an entire frequency band requested by the system could be used in a wireless handset, the volume occupied by the antenna could be reduced, and the wireless handset could be miniaturized.
An object of the present invention is to provide a novel wireless handset that can be provided with a compact antenna with a narrow bandwidth by making it possible to provide control so as to tune a center frequency of impedance matching of a tunable antenna to a frequency used for a call.
The above described problem of the present invention can be effectively solved by providing a wireless handset with a built-in tunable antenna, comprising a built-in antenna provided within a case of the wireless handset, an RF circuit part connected to the built-in antenna, a logic circuits part connected to the RF circuit part, and a frequency synthesizer connected between the logic circuits part and said RF circuit, which generates a local oscillation frequency signal in said frequency synthesizer in accordance with a first control signal from a central processing unit contained in said logic circuits part and performs sending/receiving operations with a frequency determined by said local oscillation frequency signal in said RF circuit, wherein said built-in antenna is a tunable antenna including a control circuit for center frequency of impedance matching and a control signal generator is provided within said central processing unit or in the outside connected to the central processing unit and is connected to said control circuit for center frequency of impedance matching, and wherein the control signal generator generates a second control signal from said first control signal sent to said frequency synthesizer or data used in said central processing unit to generate the first control signal, and controls a center frequency of impedance matching of said tunable antenna by applying said second control signal to said control circuit for center frequency of impedance matching.
If such means are adopted, since the first control signal or data used in the central processing unit to generate the first control signal has call frequency information determined by the central processing unit, a center frequency of impedance matching of a tunable antenna can be tuned to a call frequency using the call frequency information.
In a wireless handset, comprising a receive-only built-in antenna, an outer antenna for sending and receiving, an RF signal switching circuit connected between said built-in antenna and said outer antenna, an RF circuit part connected to said RF signal switching circuit, a logic circuits part connected to said RF circuit part, a frequency synthesizer connected between said logical circuit and said RF circuit, and a received signal strength detector provided within said RF circuit or in the outside connected thereto and connected to said logic circuits part, which generates a local oscillation frequency signal in said frequency synthesizer in accordance with a first control signal from a central processing unit contained in said logic circuits part, performs sending/receiving operations with a frequency determined by said local oscillation frequency signal in said RF circuit, and performs diversity receiving wherein an antenna with which higher received signal strength is detected in said received signal strength detector is used for receiving when an antenna connected with said RF circuit by said RF signal switching circuit is said built-in antenna or said outer antenna, wherein said built-in antenna is a tunable antenna including a control circuit for center frequency of impedance matching and a control signal generator is provided within said central processing unit or in the outside connected to the central processing unit and is connected to said control circuit for center frequency of impedance matching, if the control signal generator generates a second control signal from said first control signal sent to said frequency synthesizer or data used in said central processing unit to generate the first control signal and controls a center frequency of impedance matching of said tunable antenna by applying said second control signal to said control circuit for center frequency of impedance matching, since a center frequency of impedance matching of a tunable antenna can be tuned to a call frequency using the call frequency information using the first control signal containing the call frequency information or data used in the central processing unit to generate the first control signal, a compact tunable antenna with a narrow bandwidth could be used as a built-in antenna.
Since a miniaturized built-in antenna allows a larger distance between it and an outer antenna, the amount of electromagnetic coupling between an outer antenna and an internal antenna can be reduced, reduction of gain of both antennas can be avoided, and diversity receiving effects can be improved as a result of a reduced correlation between both antennas.
By constructing a tunable antenna used in a wireless handset according to the present invention so that it is a tunable slot antenna comprising a conductive flat cubic which is cuboid as a whole, a slim strip conductor disposed along with the direction of the resonant axis of internal space of the conductive flat cubic and in insulation from the conductive flat cubic, a slot for sending and receiving radio waves, formed across the strip conductor on the top of the conductive flat cubic, and a slip island conductor disposed in insulation from the conductive flat cubic within the slot, wherein RF power is supplied between a coupling part set in said strip conductor and the wall face of said conductive flat cubic, and wherein a variable capacitance circuit connected between said island conductor and the wall face of said conductive flat cubic is provided as said control circuit for center frequency of impedance matching, since the antenna has single-side directivity, parts can be installed on the circuit board whose face is opposite to a face on which the slot of the antenna is formed, and the packaging density can be increased, so that a wireless handset can be made more compact.